(1) Field of the Invention
The present invention relates to the continuous casting of steel and particularly to the problems of high residence time steel exiting the outlet of a refractory vessel and increased likelihood of clogging, and the deposition of nonmetallic inclusions at the outlet of a refractory vessel. The invention is configured to prevent vortex tubes from reaching the outlet and carrying slag to the outlet, and introduces controlled turbulence in the outlet to delay the deposition of nonmetallic inclusions. The invention is also configured to combine cold steel at the bottom of a refractory vessel, in a controlled manner, with steel in the body of the vessel to homogenize the temperature of steel exiting from the vessel and to avoid clogging produced by the passage of an excessive proportion of cold steel. In particular, the invention relates to a refractory piece that modifies the liquid steel flow inside a refractory vessel as the flow is directed towards the outlet. The refractory piece may achieve these effects in conjunction with a stopper. The invention also relates to an assembly comprising a refractory piece as described previously, in conjunction with a stopper. The stopper may have a rippled exterior; the ripples may form concentric rings on the end of the stopper in proximity to the outlet of the refractory vessel.
With growing demands for quality and property control, cleanliness of steel becomes more and more important. Issues like controlling the chemical composition and the homogeneity remain important, but have been joined by concerns generated by the presence of non-metallic inclusions and by clogging.
The presence of aluminum oxide and spinel inclusions is considered as harmful both for the production process itself as for the steel properties. These inclusions are mainly formed during the deoxidation of the steel in the ladle, which is necessary for continuous casting. Incomplete removal of the non-metallic inclusions during secondary metallurgy and reoxidation of the steel melt cause nozzle clogging during continuous casting. The layer of clogged material contains generally large clusters of aluminum oxide. Its thickness is related to the amount of steel cast as well as to the cleanliness of the steel. Nozzle clogging results in a decreased productivity, because less steel can be cast per unit of time (as result of the decreasing diameter) and due to replacement of nozzles with concurrent casting interruptions. Besides clogging, the presence of reoxidation products may give rise to erosion of the nozzle and to the formation of inclusion defects in the steel.
Clogging can also be produced by the entrainment of materials at or near the surface of the molten metal (e.g., slag) in the molten metal itself. Transferring molten metal from a metallurgical vessel also involves the separation of an impurity containing slag (the supernatant light phase) from a refined or partly refined metal (steel) below. As the flow from the vessel takes place, it is not uncommon for a funnel or vortex to be created which can entrain large amounts of slag into the flow of the liquid metal with resulting metal quality problems downstream.
(2) Description of Related Art
Flow behavior in an emptying vessel is influenced by the rotational velocity components in the liquid. In the absence of such velocity components, liquid leaving the emptying vessel is drawn mainly from a hem i-spheroidal region surrounding the exit nozzle, and surface liquid far above the drainage nozzle shows little motion. Toward the very end of the drainage, entrainment of the supernatant fluid does occur as a non-vortexing funnel through a funnel-shaped core.
It would therefore be desirable to provide a solution which would produce the homogenization of the temperature of molten steel passing through the outlet of a refractory vessel, and the reduction or delay of the deposition of nonmetallic inclusions in or below the outlet, while avoiding vortexing and entrainment of supernatant fluid in the exit flow form the refractory vessel.